Tundishes and ladles are intermediate containment vessels used in processing metals and metal alloys. These vessels contain a permanent refractory lining material, which is resistant to high temperatures. Typically, these permanent linings are formed from bricks or castables, and comprise 50 to 70% Al.sub.2 O.sub.3. Although these permanent lining materials are highly resistant to elevated temperatures, contact with molten metal and slag, and numerous heating and cooling cycles during the processing of molten metals can degrade the permanent liner, so that frequent replacement of the permanent liner is required. Therefore, disposable liners, formed of dry vibratable, trowellable, gunnable, or sprayable refractory materials are formed upon the permanent liner of a tundish or other molten metal processing vessel to extend the useful life of the permanent liner.
The disposable lining acts as a thermal and chemical barrier, which protects the permanent liner, and, simultaneously, maintains the temperature of the molten metal. After one or more heats of molten steel are put through the tundish, the disposable liner must be replaced. As the disposable lining material has a coefficient of thermal expansion that differs from that of the permanent liner, it would be expected that "deskulling" or removal of the disposable liner would be easily accomplished. However, alkali oxides, such as, for example, Na.sub.2 O and K.sub.2 O, often found in prior art refractory compositions, can react with the permanent lining material at elevated temperatures, and cause the two liners to fuse. As a result, deskulling of the disposable liner becomes difficult, and the permanent lining can be damaged during the deskulling process. In addition, contamination of the permanent liner caused by the fusion of the two liners increases the coefficient of thermal expansion of the surface of the permanent liner, and the difference in the rate of expansion of the surface and the remainder of the liner can result in sheet spalling and premature failure.
Water is generally added to the disposable lining composition in order to facilitate application by trowelling, gunning, or spraying. Sprayable prior art refractory compositions typically contain about 20 to 30 percent water, so that they may be pumped through a hose and sprayed when mixed with compressed air in a spray gun. However, sprayable refractory compositions can suffer from slumping, which is a condition in which a recently applied refractory material lacks the necessary adhesion to remain fixed to the surface to which it is applied. In addition, these sprayable refractory compositions often develop cracks during air drying or curing, increasing the surface area available to corrosion by molten slag and metal.
Substantially alkali metal-free, basic refractory materials, such as those disclosed in U.S. Pat. No. 5,302,563, the content of which is expressly incorporated herein by reference thereto, have been used to overcome these problems. Here, the term basic refers to the chemical properties of these materials, rather than the complexity of their compositions. Basic refractory materials include magnesia, magnesite, doloma, dolomite, olivine, calcia, or mixtures thereof. In addition to the refractory aggregate, these refractory compositions often include various binders, density reducing filler materials, and plasticizers, as well as a stiffening agent and a set retarder that can be added in predetermined weight ratios to achieve optimal performance during spray application of the composition.
The refractory compounds disclosed in U.S. Pat. No. 5,302,563 provide low density, substantially alkali metal oxide free refractory compositions, which contain less than about 0.1 weight percent alkali metal oxides, resist slumping when applied by spraying, while simultaneously providing a residence time of up to about 30 to 40 minutes without blocking the spray apparatus. The dried density of these compositions is below 100 pounds per cubic foot ("p.c.f."), and is typically between about 80 and 95 p.c.f. Also, the alkali metal containing components of these compositions total no more than about 0.1% of the overall composition, and disposable linings formed from these refractory compositions enhance the longevity of the permanent linings within the casting vessel by limiting the fusion of the disposable liner with the surface of the permanent lining.
The density reducing filler materials, previously added to control the density of the lining composition, include inorganic or organic fibers, expanded inorganic or organic materials, or other lightweight filler materials. Typically, coarse or fine paper fibers, rockwool, glass fibers, expanded clay, and expanded polystyrene beads are used. However, these materials can compromise the strength and chemical durability of the lining. Filler materials used in the prior art often form voids in the lining material that allows slag and molten metal to enter pores in the surface of the lining, where it attacks the lining material and causes corrosion of the liner surface. If the filler material contains fibers, an interconnected porosity can result, which can increase the rate of corrosion. Polystyrene beads, which decompose on heating, leaving only residual carbon, are an improvement over fibrous filler materials, because a continuous pore network is not formed. However, when these beads decompose, spherical voids form which are interconnected with other pores in the lining material.
Hollow, ceramic microspheres, by their structure, are also voids, but these voids are encapsulated by a solid ceramic shell that isolates the voids from the interconnected porosity network. As a result, lining material strength and durability are not compromised. U.S. Pat. Nos. 4,022,358, 4,874,726, and 5,252,526 disclose microspheres with a particle size of about 1 to 350 microns as a density reducing filler material for alumina and silica based refractory materials. These hollow, ceramic microspheres, available commercially under the name Fillite, are stronger and have better thermal insulation properties than other density reducing filler materials, and are highly compatible with silica and alumina based refractory materials. However, ceramic microspheres have not been used previously with basic refractory materials, such as magnesia, because of the anticipated problems of corrosion and the formation of low melting compounds.